Materials which fluoresce when exposed to UV light are well known in the art. Such fluorescent materials absorb UV radiation of one wavelength and emit UV or thermal radiation at a longer wavelength. Fluorescent materials are finding many uses in the fight against counterfeiting by providing a means for imprinting or tagging objects or materials in a manner which is only visible when illuminated by UV light.
Also known in the art is the effect of Fluorescence Resonance Energy Transfer (FRET) where UV light is absorbed by a first fluorescing material which emits radiation at a wavelength within the absorption spectrum of a second fluorescing material. Depending on the type of fluorescing materials all or a portion of the radiation emitted by the first fluorescing material is absorbed by the second fluorescing material, which in turn emits radiation at a third longer wavelength.
Prior art documents which discuss various aspects of such materials and their use are U.S. Pat. No. 3,801,782 by Dorion, the international application published with the number WO 98/22291 by Cyr, et al., and Canadian Patent Application No. 2,449,171 by Smuk, et al. the entire contents of which are incorporated herein by reference.